Evidence for X-Ray Emission from a Large-Scale Filament of Galaxies?

Cosmological simulations predict that a large fraction of the baryonic mass of the universe exists as 105-107 K diffuse, X-ray-emitting gas, tracing low-density filament and sheetlike structures exterior to massive clusters of galaxies. If present, this gas helps reconcile the current shortfall in observed baryon counts relative to the predictions of the standard big bang model. We present here the discovery and analysis of a 5 sigma significance half-degree filamentary structure, which is present in both the I-band galaxy surface density and the unresolved X-ray emission in a deep ROSAT PSPC field. The estimated diffuse X-ray emission component of this structure has a surface brightness of approximately 1.6x10-16 ergs s-1 cm-2 arcmin-2 (0.5-2 keV), comparable to the predictions for intercluster gas, and may represent a direct detection of this currently unconfirmed baryonic component.